Integrated circuit devices are typically electronically packaged by mounting one or more integrated circuit (IC) chips or dies to a substrate, sometimes referred to as a carrier. In a flip chip assembly or package, the die is "bumped" with solder to form a plurality of discrete solder balls over metal contacts on the surface of the die. The chip is then turned upside down or "flipped" so that the device side or face of the IC die can be mounted to a substrate having a corresponding array of metal contacts. Typically, the metal contacts of the substrate are coated with a solder alloy. Electrical interconnection of the die to the substrate is conventionally performed by aligning the die to the substrate and reflowing the solder on the die and/or the substrate to electrically and mechanically join the parts. Directly coupling the die immediately below the substrate allows for an increased number of interconnections and improves voltage noise margins and signal speed.
Typically, a flux composition is applied to either the die or the substrate to facilitate the formation of the interconnect. Flux acts as an adhesive to hold the placed components in place pending soldering and further acts to minimize metallic oxidation that occurs at soldering temperatures thereby improving the electrical and mechanical interconnection and reliability between the soldered component and substrate.
Soldering fluxes fall into three broad categories: rosin fluxes, water-soluble fluxes, and no-clean fluxes. Rosin fluxes, which have a relatively long history of use and are still widely used in the electronics industry. Water-soluble fluxes, which are a more recent development and which are increasingly used in consumer electronics, are highly corrosive materials. No-clean fluxes, a very recent development, reportedly do not require removal from the circuit assemblies. The most common flux for IC die attach packaging comprises a suspension liquid of various acids suspended in an alcohol base.
It has been observed that controlling the amount of applied flux is important irrespective of the type of flux employed in a particular packaging process, since enough flux must be used to effect a reliable metallurgical bond to electrically and mechanically interconnect the component to the substrate. Too much applied flux, however, can undesirably cause displacement of the placed component due to flux boiling. Excess flux further adversely impacts other circuit board manufacturing processes. For example, traces of the soldering flux residues which remain after solder reflow can lead to circuit failure, delamination of underfill, etc.
Accordingly, a continual need exists for improved processes and/or assemblies for the packaging of electronic components on to substrates employing solder fluxes.